92GT For 66-68

[hehhehmule]

I'm assuming you mean engine codes. Just remember "You can have you're CAKES but you can't E them"
C-289-2v
A-289-4v
K-289-HiPo
E-There is no E code, thus you can't E them
S-390.

These are applicable from 65-68 with exceptions like 260's, early 289's and 302's(F&J) in 68.You may also disregard the C-codes, as I think you had to be a 4V to be a GT, but I'm not 100% certian. It also doesn't include W or R codes which you most likely won't encounter.

Check out the floors, that's the worst problem area. If it's a 4lug wheel, it's not a GT. It should also have front disc brakes.